This invention relates to a process for preparation of polychloroprene having reduced odor and to polychloroprene compositions prepared by that process.
Polychloroprene is prepared on a commercial scale by free radical initiated emulsion polymerization of 2-chloro-1,3-butadiene (i.e. chloroprene), optionally in the presence of comonomers. Generally, the polymerization is conducted in an aqueous medium in the presence of an anionic soap system, although for certain specialty types of polychloroprene either a nonionic or cationic surfactant is used. A water soluble redox catalyst is normally employed and provides rapid reaction rates at relatively low polymerization temperatures.
In practice, chloroprene monomer and any optional comonomers are added to an aqueous phase containing a surfactant. The mixture is then subjected to high shear, which causes the monomer droplets to be reduced to a few microns in size. Surfactant and monomer distribute between the monomer droplets and surfactant particles (micelles) that are suspended in the aqueous medium. Initiator and optional additives, for example sulfur or chain transfer agents such as mercaptans or dialkyl xanthogen disulfides, are also present in the aqueous or organic phase. Free radicals formed in the aqueous phase migrate to the micelles, where polymerization takes place. When monomer conversion has reached the desired level, a shortstop agent is added which terminates the polymerization reaction. The resultant polymer composition is a polychioroprene latex comprising polymer dispersed in water.
By-products, residual additives and monomers may be removed from the latex by steam stripping. During productionlof dry polychloroprene, additional quantities of these residual additives are removed or reduced during isolation of the dry polymer, for example by water washing polymer film isolated using the freeze roll process and then drying. Such processes are disclosed in U.S. Pat. Nos. 2,467,769 (Morrow et al.); 2,187,146 (Calcott et al.); and 2,384,277 (Calcott et
The presence of by-products in polychloroprene latex, for example chloroprene dimers and other unidentified low molecular weight materials, can have an undesirable effect on the odor and physical properties of elastomeric materials prepared from the latex. Even rigorously stripped latexes can retain an undesirable odor. In the past, various methods have been utilized to reduce the odor of polychloroprene latex or solid polychloroprene, for example by addition of reodorants such as vanillan, o wintergreen, terpineol, anisic aldehyde, oil of cloves, camphor, and coumarin, as disclosed in J. C. Carl, Neoprene Latex, 67, E. I. du Pont de Nemours and Co. (1962). Addition of reodorants does not eliminate undesirable odor in the finished product, but rather masks it.
The presence of minor odors associated with polychloroprene latexes has caused these products to be excluded from use in a number of product formulations, even though performance properties are excellent. For example, polychloroprenes are not utilized in manufacture of adhesives for certain food packaging applications due to odor and taste problems caused by the presence of residual reodorants. An effective process for eliminating taste and odor-producing components of polychloroprene without addition of reodorants would permit use of high performance polychloroprene adhesives in food packaging applications.
The present invention is directed to a process for producing a polychloroprene composition of low odor which comprises mixing a stabilized polychloroprene latex having a chloroprene monomer content of 0.8 weight percent or less with a compound selected fro m the group consisting of ozone, hydrogen peroxide, alkali metal salts of halogen oxo acids, alkali metal salts of peroxy acids and mixtures thereof
The present invention is further directed to a polychloroprene latex of low odor prepared by a process which comprises mixing a stabilized polychloroprene latex having a chloroprene monomer content of 0.8 weight percent or less with a compound selected from the group consisting of ozone, hydrogen peroxide, alkali metal salts of halogen oxo acids, alkali metal salts of peroxy acids and mixtures thereof.
In addition, the present invention is directed to a solid chloroprene polymer of low odor prepared by a process comprising isolating a solid chloroprene polymer from a stabilized polychloroprene latex, wherein said stabilized polychloroprene latex is prepared by a process comprising mixing a polychloroprene latex having a chloroprene monomer content of 0.8 weight percent or less with a compound selected from the group consisting of ozone, hydrogen peroxide, alkali metal salts of halogen oxo acids, alkali metal salts of peroxy acids and mixtures thereof.
The compositions of the present invention are polychloroprenes (i.e. polymers having polymerized units of chloroprenee) characterized by low odor, low residual monomer content and excellent curing profile. The polychloroprenes can be in the form of latexes (i.e. polymer dispersed in aqueous media) or solids. The polychloroprene latexes can be used to manufacture dipped goods that exhibit many of the elastomeric properties of natural rubber, but which have lower odor and are less allergenic. The latexes may also be used to form adhesives that have low odor and provide excellent adhesion and tack. The isolated, solid form of the polymer can be used in manufacture of a variety of elastomeric articles, such as hoses, belts, gaskets, elastomeric thread, closed cell foam, and other general purpose rubber goods. The isolated polymer may also be dissolved in solvents to form adhesive compositions.
The compositions of the invention are prepared by a process that comprises treating a stabilized polychloroprene latex with an oxygenating substance that reduces its odor without compromising important physical properties, such as elongation at break. By xe2x80x9cstabilizedxe2x80x9d is meant that free radical polymerization reactions in the polychloroprene latex have been terminated, for example by treatment with a short-stopping agent. Stabilized latexes that have been treated with oxygenating agents according to the present invention are characterized by marked reduction in odor compared to freshly prepared, untreated latexes. In addition, films prepared from samples of treated latex have a reduced odor when compared to films prepared from untreated latex. Such solid polychloroprene compositions of the invention are prepared from the treated latex using techniques such as coagulation, extrusion, or freeze roll isolation.
Polychloroprene latex is produced in a free radical emulsion polymerization reaction. The polymerization reaction is most commonly carried out in aqueous emulsion at temperatures of between 0C. to 80xc2x0 C., in the presence of initiators such as potassium persulfate or cumene hydroperoxide. In a first step, chloroprene monomer or a mixture of chloroprene monomer and any optional copolymerizable monomers is added to a stirred reaction vessel. Such copolymerizable comonomers include, but are not limited to styrene; the vinyltoluenes and vinylnaphthalenes; 1,3-butadiene; isoprene; 2,3-dimethyl-1,3-butadiene; 2,3-dichloro-1,3-butadiene; methyl vinyl ether; vinyl acetate; methyl vinyl ketone; acrylic and methacrylic acids; ethyl acrylate; methyl methacrylate; methacrylamide; and acrylonitrile. Any such comonomer present will be added in an amount that will result in incorporation of up to 65 weight percent of one or more of the comonomers in the polymer produced during the polymerization reaction. That is, the polychloroprene component of the latex produced will comprise at least 35 weight percent copolymerized units of chloroprene. Generally, the amount of comonomer or comonomers used will be an amount that results in incorporation of up to 20 weight percent copolymerized units in the isolated, solid polymer. However, useful polychloroprene latexes can comprise chloroprene copolymers that contain up to 65 weight percent comonomer or comonomers, for example a copolymer latex wherein the copolymer is a dipolymer of chloroprene and 2,3-dichloro-1,3-butadiene. The amount of comonomer or comonomers added will vary depending on the comonomer reactivity and the amount of incorporation desired. The polymerization may take place in the presence of element sulfur, in which case the polymer produced will contain polysulfide linkages. Alternatively, the polymerization may take place in the presence of chain transfer , such as dialkyl xanthogen disulfides, dialkoxyxanthogen disulfides, iodoforr alkyl mercaptans. As is well known in the art, polymerization can be carried to a predetermined desired degree and stopped by use of conventional xe2x80x9cshort-stoppingxe2x80x9d agents. Such agents include phenothiazine, hindered phenols, or other radical trapping agents. Generally, monomer conversion is from about 50%-100%. Preferably, monomer conversion will be from 60%-98%. Emulsifying agents will also be present in the polymerization mixture.
Unreacted monomer can be rem oved by conventional procedures such as, for example, steam stripping. The level of chloroprene monomer in stabilized polychloroprene latex can usually be reduced to below 0.8 weight percent, preferably below 0.5 weight percent, most preferably below 0.1 weight percent by this procedure. In some instances, polymerization conditions and high conversion will result in production of latexes having residual chloroprene monomer levels below 0.8 weight percent. In such cases, a stripping step is not required.
Following termination of the polymerization reaction and optional stripping of the polymer latex, an oxygenating compound is added to the latex. The latex will have a chloroprene monomer level of 0.8 wt. percent or less at this point. The oxygenating compound is selected from the group consisting of ozone, hydrogen peroxide, alkali metal salts of halogen oxo acids, alkali metal salts of peroxy acids and mixtures thereof. Examples of halogen oxo acids are FOH, ClOH, HClO2, HClO3, HClO4, BrOH, HBrO2, HBrO3, HBrO4, IOH, and HIO4. Examples of peroxy acids include percarbonic acid, perboric acid and persulfuric acid. Preferably, the alkali metal will be sodium or potassium. Preferred oxygenating compounds include sodium bromate, sodium percarbonate, and sodium perborate. Especially preferred oxygenating compounds are ozone, sodium hypochlorite and hydrogen peroxide because they are readily available and cost effective compared to other agents. The oxygenating compounds may be added to polychloroprene latex in the form of solids, gases, liquids or solutions, depending on the agent selected. Generally 0.01 to 5.0 parts by weight per 100 parts by weight polychloroprene latex of oxygenating compound is used and the 0.01 to 0.5 parts by weight refers to the amount of active ingredient of a liquid or solid oxygenating compound, rather than/the amount of an aqueous solution of the oxygenating agent. Ozone may be bubbled into the latex as a gas. Preferably, such addition of ozone will take place over a cumulative period of 20 minutes to 24 hours, depending on the quantity of latex to be treated and ozone-generating apparatus used, in approximately 5 minute intervals, to permit any foam produced to settle. Hydrogen peroxide is preferably added as an aqueous solution.
Preferably between about 0.025-1.0 parts by weight of the liquid or solid oxygenating compounds per 100 parts by weight polychloroprene latex will be added. Generally, the higher the level of oxygenating compound, the greater the degree of odor reduction. At levels above about 5.0 parts by weight of oxygenating compound per 100 parts by weight of polychloroprene latex the deodorizing effect does not increase appreciably and therefore the use of higher levels is not economically efficient.
The concentration of ozone generated by some ozone-generating apparatuses is difficult to control. It has been found that, in general, if kilogram quantities of polychloroprene latexes are treated for a period of from 20-60 minutes with ozone from an ozone bubbler, such as a commercial pool or spa ozonater, then a marked reduction in odor of the latex, and polymers isolated therefrom, results. Larger volumes of latex may require longer treatment periods to reach the same level of odor reduction. For example, treatment of a 10,000-20,000 liter quantity of latex with a single ozone source at low bubble rates with good agitation may require approximately 8-10 hours to achieve a significant degree of odor reduction. Alternatively, treatment for 1 hour with multiple small ozone sources and excellent agitation would be an effective method of treatment. In either case, it is preferable to introduce the ozone in short time segments, interspersed with time for any foam to settle.
The oxygenating agents useful in the practice of the invention are effective in reducing odor associated with polychloroprene latex when 0.8 weight percent or less of residual chloroprene monomer is present in the latex to be treated. When levels of chloroprene monomer above 0.8 weight percent are present, some agents initiate polymerization. For example, potassium persulfate is commonly used as a chloroprene polymerization initiator. It also is effective at reducing odor in isolated, stripped polychloroprene latex. Consequently, addition of oxygenating compounds for purposes of odor reduction should be made at a point where the level of chloroprene monomer has been reduced to 0.8 weight percent or less, preferably 0.5 weight percent or less, most preferably 0.1 weight percent or less.
Odor reduction is difficult to quantify, but a measure of odor reduction can generally be obtained by allowing polychloroprene films isolated from both untreated latexes and the treated latexes of the invention to equilibrate in separate sealed containers for a 12 hour period and then comparing the odor of the various samples. It is usually possible for individuals to distinguish between films that have a strong odor and those having a reduced odor to the extent that odor can be distinguished and rated on a scale of 1-4 or 1-5 for strongest to least odor. Semi- quantitative results can be obtained using gas chromatographic methods. For example, the Aromatrax(trademark) system developed by Microanalytics, Inc. is a gas chromatographic method that can be used to detect odorous components of organic mixtures.
Polychloroprene latex is an article of commerce and is useful in preparation of manufactured articles such as gloves or adhesives. However, most polychloroprene is sold in the solid form (generally in the form of chips), after isolation from the latex. Isolation of the solid polymer is generally performed using a freeze roll process wherein a film of the polymer is isolated, washed, dried, and then cut into chips.
It has been found that, in comparison to polychloroprenes that have not been treated with an oxygenating compound of the above-described types, the treated polychloroprenes of the invention are markedly lower in odor. Odor in polychloroprene has been attributed to the presence of chloroprene dimers and other by-products of the polymerization process. Whatever its source, treatment of the latex with oxygenating compounds has been found to decrease the odor of polymer film prepared from polychloroprene latex. Although the precise mechanism of odor reduction accomplished by the process of the invention is not established, the presence of reactive oxygen atoms in the group of compounds suitable for use in the invention suggests that the compounds react with impurities or by-products present in the latex to form materials of low odor and taste.
A further benefit of the process of the invention is that treatment with oxygenating compounds improves certain physical properties of polychloroprene isolated from the treated latexes. In particular, the curing characteristics of the polychloroprenes may be customized by selecting an appropriate oxygenating compound according to the process of the invention. For example, polychloroprene latex suitable for use in preparation of solvent-borne adhesives may be treated with sodium hypochlorite to improve cure rate of isocyanate- based cures. If a more stable adhesive is desired, it may be treated with ozone to retard isocyanate cure rate. In other applications, improved tensile strength at higher elongation is obtained using latexes prepared by the process of the present invention.
Advantages of the process of the invention include the ready availability of the oxygenating compounds and their effectiveness at low levels. Because the perception of odor and taste is highly subjective, the level of oxygenating compound added can be easily modified to fit the level of odor reduction necessary.
The treated latexes of the present invention are particularly useful for preparation of adhesive compositions for use in food packaging. They are also useful in manufacture of gloves and other dipped goods, balloons, and blood pressure cuffs. Gloves and balloons are of particular interest as replacements for natural rubber. Gloves made from natural rubber latex are known to cause allergic reactions in up to 25% of medical professionals. Also, hospitals are creating natural rubber free zones for sensitized workers and patients. The treated polychloroprene latexes and solid polymers derived therefrom are also suitable for use in applications such as binders, coatings, foams and general rubber articles, such as hoses, belts, foam mattresses, elastomeric thread, tubing, and wet suits.